Saturday, January 17, 2009

Author Caitlín R. Kiernan considers the science of putting zeppelins on Mars

Eagerly awaiting the return of “Bradbury Weather,” a novella by Caitlín R. Kiernan that was first published in Subterranean magazine issue #2 (2005) and will be reprinted in her forthcoming collection A is for Alien (Subterranean Press, February 2009), I’ve been reading through Kiernan's Low Red Moon journal, which spans the years 2001 to 2005. While she reveals that “the best science-fiction authors recognize that the science must be peripheral to the heart of a story,” Kiernan also believes “a science-fiction writer has an obligation to get the science as ‘right’ as she can at the point in time she's writing a story."
Wednesday, September 01, 2004 ...

I spent several hours researching zeppelins, Martian aerodynamics, hydrazine, nitrogen tetroxide oxidizers, entomopters, and the problems one encounters with propellers and rotors in a thin atmosphere. Turns out, putting zeps on Mars is not as easy as I'd hoped (but nothing ever is). Consider the following:

On Mars, with a sea level equivalent pressure of only 0.7 percent that of Earth, a ten-foot cube of hydrogen would weigh about seven one thousandths as much as on Earth, or about 3.5 thousandths of a pound. But even the Martian atmosphere, at a near vacuum, only weighs in at about a tenth of a pound. So the net difference in weight would be about ninety-six and a half thousandths of a pound. This means that to get a full 73 pounds of lift, we would need about 760 such cubes. Fortunately, Martian gravity is only thirty seven percent that of Earth. So we need even fewer cubes, about 280 cubes. So to carry the same payload on Mars as on Earth we are looking at a design that begins almost 300 times as large as a similar vehicle on Earth [italics mine - CRK]. This sounds extreme, but amounts to a cube of hydrogen on Mars of 67 feet on a side producing our net 27 pounds of lift. Ignoring such pesky add-ons such as structural weight, a dirigible made to lift one person of 200 Earth pounds, or 74 Martian pounds, would need about three Mars-sized cubes for lift. Four people would need a dozen, plus another dozen for payload, and another couple of dozen for fuel and structure. This means a spherical balloon would need to hold almost 50 volumes of a third of a million cubic feet each to be useful. A dirigible of 17 million cubic feet is called for, about triple the size of the Hindenburg.

And I need zeps that can carry dozens of people and a significant cargo payload.
Pictured above: Subterranean #2, depicting a zeppelin on Mars.

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